This invention relates to a power factor monitoring and control system for resistance welding with line disturbance immunity, and more particularly, to a control which senses the resistive changes that occur in the formation of a quality weld in the secondary of a welding transformer by monitoring the corresponding change in the total load power factor as sensed in the primary of a welding transformer to provide an automatic heat control for improving weld quality over the tip life as well as reducing power consumption.
Resistance welding is now widely used in most applications involving the joining of metal, such as mild steels used in the manufacture of automobiles, and is recognized by all users as a most economic and rapid process when properly applied. Nevertheless, there are a number of parameters in the weld process that must be monitored in order to provide a quality weld. One such parameter that must be carefully monitored is the wear (mushrooming) of the welding electrodes (tips) which must be compensated for by increasing the weld heat in order to insure a good quality weld throughout the tip life.
Typically, prior art attempts at insuring weld quality despite tip wear included the following monitoring (feedback) control techniques:
1. Optical (infra-red), i.e., monitoring surface radiation to assess weld quality;
2. Weld expansion, i.e., monitoring electrode displacement (thermal-expansion);
3. Ultra-sonic, i.e., monitoring ultra-sonic transmissions transmitted through the weld area during the weld formation process;
4. Weld energy, i.e., monitoring weld energy during the formation of the weld;
5. Acoustic emmissions (expulsion detector), i.e., monitoring the acoustic emmisions during weld formation; and
6. Resistance change, i.e., monitoring the resistance change occurring during the weld formation.
Specifically, all of the above prior art techniques of monitoring weld quality were implemented by attaching or positioning various sensors and their respective leads in direct contact with the welding electrodes or in close proximity thereto. Unfortunately, these monitoring devices and their leads attach to, or in close proximity to, the welding electrodes, that work so well in a laboratory environment when manned by expert technicians, seldom stood up in an industrial environment in which welding machines are sometimes manned by unskilled operators on an assembly line. The results, in many cases, were damaged monitoring devices as well as severed leads thereto which made it impossible to monitor the quality of the weld.
For the above stated reasons, monitoring devices and their leads attached to or in close proximity to the welding electrodes are often inadequate to assure good quality welds throughout the tip life because of the continual maintenance problems. Examples of the above type of monitoring techniques and associated devices are contained in a publication entitled "Resistance Welding Control Monitoring" published by the Welding Institute located at Abington Hall, Abington, Cambridge, C.B. 16Al, United Kingdom, copyrighted 1977.
One successful way to compensate for electrode wear without attaching monitoring devices next to or on the welding tips is found in the Digital Welder Control System of U.S. Pat. No. 4,104,724 ('724 Patent). The controller of this patent provided a maintenance interval counter and compensator control having a 4-step, stepper. The stepper control of this patent is used to automatically increase the weld heat after a preset number of welds based on past experience to compensate for electrode mushrooming. Moreover, the digital welding control system of this patent is hereby incorporated by reference as to a type of digital welder control system that is ideally suited for modification to incorporate the features of the present invention.
The invention disclosed here represents an improvement over that disclosed in the patent application Ser. No. 006,990 entitled "Power Factor Monitoring and Control System for Resistive Welding" filed Jan. 29, 1979. It has been found that in situations where several welders are operating simultaneously on the same power bus line, the line voltage waveform becomes distorted so that the line voltage zero crossing can no longer be used as an accurate timing reference point. Additionally, the distortion of the voltage waveform has an effect on the current extinction angle. Both of these problems can, in some circumstances, exceed the magnitude of the signal caused by the changes in resistance of the work pieces as the weld is being performed.
One feature of the present invention avoids the use of the line voltage zero crossing as a timing reference point, and substitutes, therefore, the total conduction time of the current as the measurement interval. Since the line voltage waveform distortion tends to be random in nature, the effect of this distortion can be reduced by averaging the results of the measurements over a preselected number of welds. Although this technique is incompatible with a mode of operation whereby the current is terminated as each weld is found to be complete, it does provide a basis for developing a feedback signal which can be used to control the weld current or time over a group of welds. Thus, the welder control system of the present invention adjusts itself for changes due to tip wear, line voltage, initial setpoint errors, etc. These adjustments are necessarily gradual in nature because of averaging process.